Golf club shaft with superelastic tensioning device

ABSTRACT

A shaft for a golf club or other sporting equipment is disclosed, wherein the shaft is hollow and contains a wire or cable placed under tension therein, the wire being made of a superelastic material. The wire is connected at one end to a variation device such as a cam which varies the tension on the wire and thus the bending stiffness of the golf club. Because the wire is made of a superelastic material, for example Nitinol, it can reversibly elongate in response to pre-tensioning and dynamic stresses encountered during swinging the golf club, in order to counterbalance and accommodate, the stress encountered during normal use of the golf club, thus ensuring a long life and preventing damage to the golf club shaft.

FIELD OF INVENTION

[0001] The present invention relates to shape memory alloys, which arematerials capable of recovering their original shape after beingdeformed under stress, and more particularly relates to the use of suchmaterials in sporting equipment such as a golf club or a hockey stick.

BACKGROUND OF THE INVENTION

[0002] Shape memory alloys (SMAs) are metal alloy materials that havethe ability to return to their original shape after being deformed. AllSMAs have two distinct crystal structures, or phases, with the phasepresent being dependent on the temperature and the amount of stressapplied to the SMA. The two phases are martensite, which exists at lowertemperatures, and austenite at higher temperatures. The exact structureof these two phases depends on the type of SMA, where the most commonlyused type is called Nitinol. Nitinol is a mixture of two componentmetals, nickel (Ni) and titanium (Ti), which are mixed in an approximateratio of 55% by weight Ni and 45% by weight Ti, and annealed to form apart in the desired shape.

[0003] Shape memory alloys possess two material properties that worktogether to provide shape memory. The first material property is anaustenite to martensite transition in the SMA. This is a solid-to-solidphase transition from an austenite phase with high symmetry (such as acubic molecular structure) to a martensite phase with lower symmetry(such as tetragonal or monoclinic structures). The second property of ashape memory alloy is the ability of the low-symmetry martensitestructure to be deformed by twin boundary motion. A twin boundary is aplane of mirror symmetry in the material. If the twin boundary ismobile, as in certain martensite structures, the motion of the boundarycan cause the crystal to rearrange and thus accommodate strain.

[0004] The coupling of the above two properties produces two distincttypes of mechanical behavior in shape memory alloys. These two behaviorsare referred to as “shape memory effect” and “superelasticity.”

[0005] The shape memory effect occurs when deformation incurred in themartensite phase via twin boundary motion is recovered by heating thematerial past a transition temperature to the high temperature austenitephase. The following three-stage model illustrates the changes undergoneby a shape change alloy according to this effect:

[0006] In stage 1, the alloy is in the austenite phase. As the alloy iscooled below the transition temperature, T_(m), the material tends toretain its original shape by inducing twin boundaries that allow thenewly deformed (stage 2) crystal structure to occupy approximately thesame volume as the stage 1 structure. Now, if stress is applied to thestructure, it can deform by twin boundary motion. The twin boundariesmove to rearrange the crystalline asymmetry to accommodate strain(thereby reaching stage 3). This rearrangement can occur in severaldirections, allowing the crystal structure to handle strain in multipledirections. Finally, when the material is re-heated, the asymmetry thatpermitted strain in the crystal structure disappears in thetransformation, and the material recovers to its original (stage 1)shape. The particular orientation of the crystal structure in stage 2 isunimportant, as the material returns to only one structure, i.e. theoriginal austenite structure of stage 1. Hence, such a material exhibitsthe shape memory effect. Thermally actuated shape memory materials suchas Nitinol (NiTi) include an elastic range of up to 8% reversibleelongation in some materials, and the yield stress is very low, thusallowing the material to deform easily in the martensite state.

[0007] Superelasticity uses the same deformation mechanisms as shapememory, but occurs without a change in temperature. Instead, thetransformation is induced by stress alone. Applied stress can overcomethe natural driving force which keeps the material at equilibrium in theaustenite phase. By applying stress to the material, it can be convertedinto the martensite phase, and the crystal structure will strain toaccommodate the applied stress. When this stress-energy is greater thanthe chemical driving force of stabilization in the austenite phase, thematerial will transform to the martensite phase and be subject to alarge amount of strain. When the stress is removed, the material returnsto its original shape in the austenite phase, since martensite cannotexist above the transition temperature. This superelastic behavior isfully reversible and does not require any change in temperature.

[0008] The full stress recovery of a superelastic material can occurwith up to approximately 8% elongation in Nitinol (NiTi). Because ofthis large elastic range, superelastic materials are used inapplications such as cardiovascular stents, mobile telephone antennas,and eyeglass frames. Superelastic materials have not previously beenused in sporting equipment such as golf clubs or hockey sticks.

[0009] U.S. Ser. No. 09/158,172, commonly owned with the presentapplication, discloses a variable stiffness shaft for use in a golfclub, and is incorporated by reference in the present disclosure. Astaught in the '172 application, a golf club includes a hollow shafthaving a wire placed under tension inside the shaft, the tension beingadjustable to a desired level. Such an invention is useful for varyingthe stiffness of the shaft to accommodate individual users and differentanticipated levels of stress. However, the internal wire is customarilymade of a material that does not have shape memory, such as steel (pianowire), titanium, aluminum, or a corrosion resistant plastic such asnylon, all of which are non-SMAs. Such materials can reversibly elongateby typically 0.33%-0.34%, and even as much as 1% for spring steel, butdynamic strains realized in a golf club shaft commonly range from 0.33%to about 1%, thus resulting in material failure. Accordingly,conventional materials are subject to damage after repeated blows on thegolf course.

SUMMARY OF THE INVENTION

[0010] A hollow shaft for use in a golf club or other sporting equipmenthaving a shaft is disclosed, wherein the shaft contains a tensioningdevice comprising a wire or cable made of a superelastic alloy. Thetension level of the wire can be varied in order to reduce the bendingstiffness of the shaft, in accordance with particular anticipated loadsor to accommodate the player's individual stroke. Initially, the wire ispre-tensioned by mechanically tightening the wire using a variationmeans. As a result of being tightened, the wire elongates byapproximately 0-7% (i.e. any amount within the wire's elastic range ofup to 8%) against the shaft stiffness, which is known as a pre-stress orpre-tension level of the wire. The shaft is pre-tensioned by an amountless than the maximum strain level of 8% in the wire, so as toaccommodate dynamically induced strain encountered during a swing andcontact with the ball.

[0011] As a result of pre-stressing, the shaft is compressed to thetension level produced by the wire elongation. By pre-compressing theshaft, the bending frequency of the shaft is reduced which reduces thenet flex rating and improves performance of the golf club. Shaftpre-compression also tends to offset centrifugally induced shaft tensionencountered during a swing, such that, upon impact of the golf club witha ball, a lower net strain level is present in the shaft as comparedwith uncompressed composite shafts. Thus, when coupled with presetstrains, strain levels present in the shaft at impact do not exceedyield and failure strains of the shaft.

[0012] During the swing, centrifugal loads of the accelerating golf clubhead mass result in approximately 50-100 pounds of dynamic tension forcebeing placed on the shaft. On top of this are short-term dynamicstresses and strains in the shaft that result from ball impact.Conventional composite golf club shafts degrade over time when used byhard swingers because the net dynamic strain (i.e. the large dynamicstrain that results from swing centripetal acceleration forces coupledwith impact dynamic strains) causes the material situated near the hoselend of the shaft to fail. By incorporating in the shaft a wire made of asuperelastic shape memory alloy, measured levels of swing andimpact-induced dynamic strain, which can reach approximately 0.33-1%,will not result in significant degradation of the shaft due to stress,as substantially all of the stress is absorbed by the wire. Whereas acomposite shaft incorporating a wire made of a conventional materialproduces large stress changes in the shaft which accompany relativelysmall changes in strain, thus resulting in premature fatigue andfailure.

[0013] Superelastic wires exhibit a large recoverable strain capability,and can recover approximately 0-8% of strain, or substantially theentire range of deformation produced in the wire. Since the superelasticwire can recover over a large strain range, even for nominal dynamicstresses above the pre-tension amount, the wire made of a superelasticalloy has superior fatigue and failure properties, and is also anextremely hard, corrosion-resistant material.

[0014] A preferred superelastic alloy is Nitinol (NiTi), which canreversibly elongate over an elastic range of up to approximately 8%,allowing the golf club to be swung repeatedly without damaging theshaft. As used herein, the terms “shape memory alloy” and “superelasticalloy” refer to a material having (i) an austenite to martensitesolid-to-solid phase transition, and (ii) an ability for the martensitestructure to be deformed by twin boundary motion. The preferredmaterials to be used in the present invention are superelastic alloys,which are further defined as materials that undergo the martensite toaustenite phase transition without a significant change in temperature.In superelastic alloys, the martensite to austenite transition occursdue to the dynamically applied stress forces which overcome the naturaldriving force that keeps the material at equilibrium in the austenitephase.

[0015] The golf club with a wire made of Nitinol incorporated in thehollow shaft can respond to each swing by returning to its originalpreset shape. The wire can reversibly elongate under strain over anelastic range of up to approximately 8% of reversible strain. The use ofa superelastic alloy in the golf club yields unexpected results in termsof high performance and long-lasting durability of the golf club. Astaught in the '172 application, by placing a wire under tension inside ahollow shaft of the club, bending stiffness of the club can be reduced,thereby improving trajectory for each stroke. However, when the internalwire is made of conventional materials, it tends to stretch whenever thetotal stress exceeds the maximum level of approximately 0.33-1%tolerated by the wire material, and the wire becomes damaged over time.Such stretching and damage is minimized by the use of a wire made of asuperelastic alloy, in accordance with the present invention. Theapplied stress produces elongation in the wire which is well within therecoverable strain of the superelastic wire and which causes onlyminimal variations in the pre-stressed wire. Thus, a golf club with ahollow shaft incorporating a superelastic wire (e.g. made of Nitinol)demonstrates high stroke performance but also a long term durability notpresent in golf club shafts made of a conventional material.

[0016] The stiffness of the hollow shaft is initially set by using avariation means to adjust the tension on a tensioning device disposed inthe hollow shaft. The tensioning device is attached to the shaft at twopoints. Thus, applying tension to the tensioning device causescompression in the shaft region between the two points. The tensioningdevice, as explained above, can be a wire or cable made of asuperelastic alloy, or a plurality of wires or cables. Many variationmeans are possible, including a cam, a pinned retractable piece, alockable lead screw (which may be adjusted by an external actuator), apump, a sleeve screw, or a “set and forget” displacement actuator. Theshaft can have one or more constraint inserts, which can be made, forexample, of low density foam, plastic, or an elastomer. The constraintinserts impede dynamic variation of the tensioning device. They can beheld in the shaft by compression fit or by adhesive. The sportingequipment can be, for example, a golf club, a tennis racket, a ski pole,a hockey stick, a baseball bat, a fishing pole, a hurling stick, alacrosse stick, or a vaulting pole.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] For a fuller understanding of the nature of the presentinvention, reference is made to the following detailed description takenin conjunction with the accompanying drawing figures wherein likereference characters denote corresponding parts throughout the severalviews and wherein:

[0018]FIG. 1 is a cross-sectional view of a preferred embodiment of agolf club shaft according to the present invention;

[0019]FIG. 2 is a cross-sectional view of another preferred embodimentof a golf club shaft;

[0020]FIG. 3 is a cross-sectional view of a further preferred embodimentof a golf club shaft; and

[0021] FIGS. 4A-4F are cross-sectional views of six alternativepreferred embodiments of a variation means according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE PREFERREDEMBODIMENT(S)

[0022] The present invention discloses a golf club or other sportingequipment with a hollow shaft inside the golf club, where a wire orcable is placed under tension, allowing the bending stiffness of thegolf club to be varied. In a particular aspect of this invention, thewire is made of a superelastic alloy, such that the wire recovers to itsoriginal pre-stressed state after a dynamically applied stress isreleased (i.e. after the club head strikes the ball). Accordingly, afterrepeated swings, the wire retains its original shape, with littlevariation from the pre-tensioned state, and does not suffer significantdamage, thus prolonging the life of the golf club.

[0023] Such a design utilizes the well known concept that a beam'sbending stiffness can be varied by preloading the beam along itslongitudinal axis. The present invention involves the use of thisprinciple to provide a variation mechanism for golf club shafts andother sporting equipment. The preloading device is the wire or cable(mentioned above) which is placed under tension in the hollow shaft of agolf club. Tensioning the wire/cable compresses the shaft along itsbending axis, reducing the effective bending stiffness as compared tothat of the unloaded shaft. An advantage of the present invention isthat it allows production of a shaft having a high torsional stiffness,enabling a rapid response to player wrist turnover action, but having alow bending stiffness, so that the player achieves the best balltrajectory for a given swing speed. A detailed explanation of the aboveprinciple is provided later in this description.

[0024]FIG. 1 illustrates a preferred embodiment of a golf club shaft 10having a butt end 12 and a hosel end 14, with a tensioning wire or cable16 secured at both ends of the shaft. A variation mechanism 18 islocated at the butt end 12 of the shaft, where it is readily accessibleto the user of the club. FIG. 2 shows an alternate preferred embodimentof the golf club shaft, further comprising a wire termination insert 20in the interior of the shaft. In this embodiment, the wire 16 extendsonly part way into the shaft, reaching from the butt end 12 to thetermination insert 20. FIG. 3 shows a further preferred embodiment ofthe golf club shaft, wherein the shaft includes a plurality ofconstraint inserts 22, made for example from low density foam, whichrestrain lateral motion of the wire 16.

[0025] The internal tensioning wire/cable 16 is preferably made of asuperelastic alloy which can reversibly elongate under an applieddynamic stress by an amount in excess of the normal expected strain ofapproximately 0.33-1% experienced in a golf club upon impact with theball. A preferred superelastic alloy is Nitinol (NiTi), which exhibitsan elastic range of up to 8% deformation, and is therefore able to fullyrecover from the expected strain experienced in a golf club. Such anelastic range provides significant advantages over conventional metalsand alloys which generally exhibit strain levels very close to theexpected level of 0.33-1%.

[0026] A problem encountered in conventional composite shaft golf clubsis that the dynamic stresses often produce unacceptable strain levels inthe wire 16, leading to premature fatigue and failure. Using suchmaterials, the wire is often pre-tensioned to approximately its elasticlimit, resulting in damage if any additional dynamic strain is appliedduring swinging. Repeated swings can lead to permanent damage of thegolf club and a reduced useful life. Use of a superelastic alloy such asNitinol enables the golf club to be swung repeatedly without damagingthe shaft. TABLE 1 Comparison of Material Properties of Nitinol WithOther Materials 304 NiTi 6061 Cast AM100A 4140 Stainless Material SMA AlIron Mg Steel Steel Wood Recoverable 8% 0.34% 0.23% 0.33% 0.33% 0.11%0.42% Strain (%) Recoverable ˜7000 147 41 137 142 15 51 Energy (J/kg)Density (kg/m³) 6450 2770 7064 1800 7830 7830 500 Modulus (GPa) 75 aus.70 90 45 205 197 12.1 28 mart. Yield Stress 600 241 250 150 675 215 13.1(MPa) Melting Temp. (° C.) 1240 582 — 540 1400 1427 —

[0027] As shown in Table 1 above, Nitinol provides properties superiorto those of conventional materials and is ideal for use in thewire/cable 16. The recoverable strain in Nitinol is approximately 8%,which is significantly greater than Al (0.34%), Mg (0.33%), and steel(0.33%). Spring steel (not shown) can recover up to approximately 1% ofrecoverable strain. Iron and stainless steel, the most common materialsfor golf club shafts, exhibit even lower percentages of recoverablestrain, and thus are not practical for use in a composite shaft, whereexpected levels of strain can produce approximately 0.33-1% elongation.Even using aluminum, magnesium, or steel in the hollow shaft 10 canresult in damage to the shaft because the recoverable strain of suchmaterials is very close to the expected strain experienced in a golfclub and can be exceeded whenever a large amount of stress is applied.Wood (0.42%) has a higher percentage of recoverable strain, but is notpractical for use in the golf shaft due to its low yield stress.

[0028] Also as demonstrated in Table 1, the recoverable energy ofNitinol is extremely high compared to conventional materials, a propertywhich stems from the ability of Nitinol and other superelastic alloys toconvert from the austenite to the martensite phase, and then return toits original shape upon returning to the austenite phase.

[0029] While the preferred material for the wire 16 is a Nitinol alloy,other superelastic alloys and shape memory alloys can be used, includingbut not limited to mixtures of: nickel and aluminum (Ni—Al), copper andzinc and another element Cu—Zn—X (where the other element X can besilicon (Si), tin (Sn), or aluminum (Al)), copper and zinc (Cu—Zn),copper and tin (Cu—Sn), copper and aluminum and nickel (Cu—Al—Ni), ironand platinum (Fe—Pt), iron and manganese and silicon (Fe—Mn—Si), ormanganese and copper (Mn—Cu).

[0030] When the wire 16 is made of one of the above superelastic alloys,e.g. Nitinol, the wire is subject to stress at three different stages:(1) when the wire is pre-tensioned by a desired amount, where thepre-tension amount is within the range of 8% maximum allowable strain soas to allow room for dynamically induced strain occurring during theswing and impact; (2) during the downswing as the result of tensionforces loaded onto the shaft by the accelerating club head; and (3) asthe golf club strikes the ball. Accordingly, the wire 16 is convertedfrom austenite to martensite during stressing, and the martensitestructure deforms by twin boundary motion in an elastic range of up toapproximately 8% elongation to accommodate the stress. After the stressis released, the wire 16 returns to its original zero-strain, austenitestate. The wire 16 is able to reversibly elongate under strain toaccommodate the full amount of stress encountered during a golf swing.

[0031] A number of wire/cable tension variation systems are suitable foruse with the invention. Several such systems are shown in FIGS. 4A-4F asapplied to a golf club. FIG. 4A depicts a cammed lever variation systemhaving four settings that span the accepted stiffness range for golfclub shafts. As a cam 30 is rotated in the direction of the arrow, thetension is increased on the wire 16, resulting in a softening of theshaft. There are four settings for the illustrated cam 30, correspondingto the four faces of the cam, each of which is at a different distancefrom an attachment point 32 of the wire. As shown, the wire tension isat its lowest setting, corresponding to the stiffest shaft setting. Itwill be apparent to those skilled in the art that this type of variationsystem is not limited to a four-faced cam, but can be used with a camhaving a different number of faces or with a continuous cam, as long asthe variation system is capable of holding its setting. The illustratedcam 30 is held in any of the four settings by pressure on the flat faceof the cam; a continuous cam can use a set screw or the like (not shown)to achieve the same end.

[0032]FIG. 4B depicts a plunger clevis/cotter pin variation systemhaving four settings that span the accepted stiffness range for golfclub shafts. In this arrangement, a plunger clevis 40 passes through aplug 42, which is attached to the butt end of the shaft 12. The clevis40 is provided with several holes 44 perpendicular to its axis, and isattached to the wire 16 at point 46. A cotter pin 48 is adapted to slideinto any of the holes, thereby varying the position of the plungerclevis 40 relative to the plug 42. It will be apparent to those skilledin the art that this type of variation system is not limited to havingfour levels, nor are the illustrated shapes of the clevis and cotter pinintended to be limiting.

[0033]FIG. 4C depicts a pneumatic or hydraulic variation system, where aconstant force is applied to a piston 50 by a working fluid 52 whichapplies a constant tension to the wire 16. A pumping mechanism 54 allowsa golfer to vary the tension by pumping working fluid 52 through aone-way valve (not shown). The tension can be relieved by opening thevalve to allow back flow. This type of system is continuously variableover a range of tensions.

[0034]FIG. 4D depicts a lead screw variation system comprising athreaded lead screw 60, a threaded lock fitting 62, and a guide 64attached to the butt end 12 of the shaft. The threads of the lead screw60 engage the lock 62, which can thus be set at any point on the lengthof the lead screw 60. The lock is held in compression fit with the guide64, allowing the lead screw to apply a tension to the wire 16. Theattachment point 66 is preferably designed not to twist the wire whenthe lead screw 60 is turned. This type of system is continuouslyvariable over a range of tensions. The lead screw can be turned by hand,or can be activated by an external actuator such as a battery poweredelectric screwdriver or the like. This type of system is continuouslyvariable over a range of tensions.

[0035]FIG. 4E depicts a threaded sleeve variation system. A plug 70 isaffixed to the butt end 12 of the shaft, the plug 70 having an outerthread. An inner threaded turn sleeve 72 is engaged with the plug 70.The turn sleeve supports a head 74, and can be adapted to slip relativeto the head at their point of contact 76. The head is connected to thewire 16 at point 78. It will be seen that rotation of the turn sleeve 72will raise or lower the head 74 to vary the wire tension over acontinuous range.

[0036]FIG. 4F depicts an active set and forget displacement actuatorvariation system. A head 80 is connected to the wire 16 at point 82, anda standard set and forget actuator 84 is inserted between the butt endof the shaft 12 and the head 80. Such actuators are commonly known inthe art, and generally are activated by connection of a separate powersource. Once the actuator has been set to the desired length (andcorresponding wire tension and shaft stiffness), the power source (notshown) can be removed for the swing. This variation system can vary thewire tension over a continuous range.

[0037] A detailed explanation will now be provided with respect tobehavior of the golf club shaft according to the present invention. Thefundamental behavior that this invention leverages is described by thegeneral, unforced, one dimensional equation of motion for a beam,

(EIw″)″+(Pw′)′+m{overscore (w)}=0   (1)

[0038] where EI is the elastic modulus times the shaft cross sectionarea moment of inertia that can vary along the length of the golf clubshaft, P is the axial compression applied to the shaft that can alsovary along the length of the shaft, m is the mass per unit length thatcan vary along the length of the shaft, and w is the lateral deflectionwhich is a function of time and position along the length of the shaft.The first term of Equation (1) relates how the beam bending stiffness isaffected by inertial loads through the second spatial derivative of theshaft internal moment (the bracketed term that includes second orderspatial derivative of the deflection) with respect to the axialcoordinate. The second term of Equation (1) is generally small and istypically neglected. The last term in Equation (1) represents inertialload (per unit length) resulting from motion, where force equals masstimes the second derivative of the deflection with respect to time.

[0039] When a beam is under compression, the compressive load reducesthe apparent bending stiffness of the beam by amplifying the lateraldeflection. This effect can be visualized by considering a ruler beingcompressed along its measurement axis by pressing opposing endstogether. When the center of the ruler is slightly perturbed laterally,the deflection is enhanced by the compression and the ruler bends.

[0040] This behavior can be seen mathematically, to first order, byassuming a deflection as a function of time, t, and position on theshaft, x, $\begin{matrix}{w = {A\quad {\sin \left( {\omega \quad t} \right)}{\sin \left( \frac{\pi \quad x}{L} \right)}}} & (2)\end{matrix}$

[0041] where ω is the natural frequency of motion, L is the shaft lengthand A is an arbitrary constant. This simplifying assumption is a goodone since Fourier theory states that any bounded analytic function canbe approximated by a series of sines and cosines. Placing thisdeflection shape into the equation of motion and assuming that thebending stiffness, EI, and mass per unit length, m, can be representedby constant averaged values over the length of the shaft, EI and m, andthat P is uniform, yields $\begin{matrix}{{\left\lbrack {{\frac{\pi^{4}}{L^{4}}\overset{\_}{E\quad I}} - {P\frac{\pi^{2}}{L^{2}}} - {\overset{\_}{m}\quad \omega^{2}}} \right\rbrack A\quad {\sin \left( {\omega \quad t} \right)}{\sin \left( \frac{\pi \quad x}{L} \right)}} = 0} & (3)\end{matrix}$

[0042] which, when solved for the natural frequency by setting thesquare bracketed term equal to zero, gives $\begin{matrix}{\omega = \sqrt{\frac{\pi^{4}\overset{\_}{E\quad I}\left( {1 - \frac{P\quad L^{2}}{\pi^{2}\overset{\_}{E\quad I}}} \right)}{m\quad L^{4}}}} & (4)\end{matrix}$

[0043] To first order, Equation (4) shows how the apparent stiffness,the numerator of the quotient under the square root sign, decreases withincreased static compression. For reference, the general buckling loadfor a uniform beam under uniform compression is $\begin{matrix}{P_{buckling} = \frac{{\alpha\pi}^{2}\overset{\_}{E\quad I}}{L^{2}}} & (5)\end{matrix}$

[0044] so that a compressive load equal to 20% of the buckling loadresults in a 20% decrease in stiffness and a corresponding 10% decreasein natural frequency.

[0045] It is important to note that the shaft load P changes during thedownswing, that is, the centrifugal force generated by the acceleratingclub head counters the precompression and results in general shaftstiffening. Without precompression, the centrifugal load would merelyserve to stiffen the dynamic behavior of the shaft, since the shaft loadP would by definition be negative.

[0046] For beams of general cross sections, moduli, mass per unitlength, and preloading that vary with the axial coordinate, expressionsanalogous to Equations (2)-(5) do not exist in closed form, and fullanalysis is necessary for accurate predictions. However, the behaviorfor more complicated geometries and loading is generally similar to thesimple case illustrated above.

[0047] This invention may be applied to metal, wood, and compositeshafts as long as the shafts are hollow or hollowed to allow theinsertion of the invention. The tensioning member may be affixed to anypoint along the shaft's length. This allows tailoring of the variablestiffness length of the shaft. For full length golf club shaftembodiments a tensioning wire/cable assembly may be integrated into thebutt and hosel ends of the shaft. The tensioning device may comprise asingle wire or cable, or multiple (twisted or untwisted) wires.

[0048] The wire can be restrained from dynamic vibration within theshaft by filling the enclosed volume, either entirely or partially, withlow density foam or a similar material. The wire can also be restrainedfrom dynamic vibration by placing form fitting inserts down the lengthof the wire and affixing them to the shaft, by glue or compressionresulting from expansion, thus allowing the wire to run freely throughthe insert while restraining its lateral motion.

[0049] Although the invention has been described in detail including thepreferred embodiments thereof, such description is for illustrativepurposes only, and it is to be understood that changes and variationsincluding improvements may be made by those skilled in the art withoutdeparting from the spirit or scope of the following claims.

What is claimed is:
 1. A sporting device having a variable stiffnessshaft, comprising: a hollow shaft; a length of a superelastic alloyplaced under tension within the hollow shaft and affixed thereto at twopoints on the shaft, the alloy capable of reversibly elongating toaccommodate an applied stress; and variation means for adjusting thetension of the tensioning device, whereby increasing the tension reducesthe bending stiffness of the shaft.
 2. The sporting device of claim 1,wherein the superelastic alloy is Nitinol.
 3. The sporting device ofclaim 1, wherein the superelastic alloy can reversibly elongate by up toapproximately 8% to accommodate the applied stress.
 4. The sportingdevice of claim 1, wherein the applied stress includes a pre-stressapplied to the length of superelastic alloy by the variation means. 5.The sporting device of claim 1, wherein the applied stress includes adynamic stress produced during a swing.
 6. The sporting device of claim1, wherein the length of superelastic alloy comprises a wire affixed atone end to the shaft, and affixed at the opposing end to the variationmeans, the variation means transmitting the tension of the wire to theshaft.
 7. The sporting device of claim 1, wherein the length ofsuperelastic alloy comprises a plurality of wires affixed at one end tothe shaft, and affixed at the opposing end to the variation means, thevariation means transmitting the tension of the wires to the shaft. 8.The sporting device of claim 1, wherein the variation means is a cam. 9.The sporting device of claim 1, wherein the variation means is a pinnedretractable piece.
 10. The sporting device of claim 1, wherein thevariation means is a lockable lead screw.
 11. The sporting device ofclaim 9, wherein the lead screw is varied by an external actuator. 12.The sporting device of claim 1, wherein the variation means is a pump.13. The sporting device of claim 1, wherein the variation means is asleeve screw.
 14. The sporting device of claim 1, wherein the variationmeans is a displacement actuator powered by an external source.
 15. Thesporting device of claim 1, and further comprising a constraint insertfor preventing dynamic vibration of the length of superelastic alloy.16. The sporting device of claim 15, wherein the constraint insertcomprises a material selected from the group consisting of low densityfoam, plastic, and elastomers.
 17. The sporting device of claim 15, andfurther comprising a plurality of discrete inserts.
 18. The sportingdevice of claim 15, wherein the constraint insert is a single constraintinsert extending substantially along the length of the shaft.
 19. Thesporting device of claim 15, wherein the constraint insert is held inthe shaft by compression.
 20. The sporting device of claim 15, whereinthe constraint insert is held in the shaft by adhesive.
 21. The sportingdevice of claim 1, wherein the sporting device is selected from thegroup consisting of a golf club, a tennis racket, a ski pole, a hockeystick, a baseball bat, a fishing pole, a hurling stick, a lacrossestick, and a vaulting pole.
 22. A golf club having a variable stiffnessshaft, comprising: a hollow shaft; a wire disposed within the hollowshaft and affixed thereto at two points on the shaft, wherein the wireis made of a superelastic alloy which reversibly elongates toaccommodate an applied stress; and variation means for varying thetension of the wire, whereby increasing the tension of the wire reducesthe bending stiffness of the shaft.
 23. The golf club of claim 22,wherein the superelastic alloy is Nitinol which can reversibly elongateby up to approximately 8%.
 24. The sporting device of claim 22, whereinthe applied stress includes a pre-stress applied to the length ofsuperelastic alloy by the variation means.
 25. The sporting device ofclaim 22, wherein the applied stress includes a dynamic stress producedduring a swing.
 24. The golf club of claim 21, wherein the superelasticalloy is selected from the group consisting of nickel and aluminum(Ni—Al); copper and zinc and another element Cu—Zn—X (where the otherelement X is silicon (Si), tin (Sn), or aluminum (Al)); copper and zinc(Cu—Zn); copper and tin (Cu—Sn); copper and aluminum and nickel(Cu—Al—Ni); iron and platinum (Fe—Pt); iron and manganese and silicon(Fe—Mn—Si); and manganese and copper (Mn—Cu).
 25. The golf club of claim21, wherein the wire is affixed at one end to the shaft, and affixed atthe opposing end to the variation means, the variation meanstransmitting the tension of the wire to the shaft.
 26. The golf club ofclaim 25, wherein the tensioning device comprises a plurality of wiresaffixed at one end to the shaft, and affixed at the opposing end to thevariation means, the variation means transmitting the tension of thewires to the shaft.
 27. The golf club of claim 21, wherein the variationmeans is a cam.